Image intensifier storage tube



R, H. ANDERSON IMAGE INTENSIFIER STORAGE TUBE Filed Aug. l2, 1965 MONITOR CRT N DORO TS NR E ED E E EVN m M MWA T W N H. 1| l R E B O R i| I|III.I.L w E F/ I R N F O O O T f s 2 E m, F m fw w w L l 11...!4 Il! E m S A R E 5 O O O o O G m m Q m l .r wPJO BY BUCKHORN, BLORE, KLARQUIST SPARKMAN ATTORNEYS nited States Patent O 3,317,782 EMAGE ENTENSIFIER STORAGE TUBE Robert H. Anderson, Portland, Greg., assigner to Tektronix, Inc., Beaverton, Greg., a corporation of Oregon Fiied Aug. 12, 1963, Ser. No. 301,399 9 Claims. (Cl. 315-11) The subject matter of the present invention relates generally to apparatus for the intensication of images formed by electro-magnetic radiation, and in particular to an image intensifier tube having a simplified bistable, direct viewing storage target which stores the charge image formed thereon by high velocity electrons corresponding to the light image received by the photocathode of such tube and which enables visual and/ or electrical readout of such store-d charge image.

The image intensifier storage tube of the present invention is especially useful as an infrared detector for night observation of military objects. However, the present intensier storage tube may also be used for detecting and storing X-ray or visible light images merely by changing the material of the photocathode employed in such tube. Alternatively, a fiuorescent screen may be positioned outside of the intensifier tube adjacent the photocathode to convert infrared, X-rays or other invisible radiation into visible light which is then directed onto a conventional visible light sensitive photocathode inside such intensifier tube. It has been found that an extremely simple and highly sensitive image intensifier tube may be produced by employing an electron image storage target in such intensifier tube. The charge image produced on the storage target Within such intensiiier tube may be stored by bombarding such target with low velocity electrons which cause secondary electrons to be emitted from such target. This storage target may include a storage dielectric layer of phosphor material coated over a thin light transparent film of conductive material coated on the inner surface of one end of the glass envelope of the tube. The phosphor dielectric layer is of a thickness which enables the storage of a charge image produced thereon and emits a visible light image corresponding to the charge image to provide a visual readout lof such charge image. The image intensifier storage tube of the present invention may also be provided with electrical readout by scanning the surface of the phosphor dielectric layer with a narrow beam of electrons emitted by a reading electron gun within such tube to pro-duce an electrical readout signal on the conductive film which corresponds to the charge image on such phosphor layer.

The simplified'structure of the storage target used in the intensifier tube of the present invention is extremely economical and produces superior results to conventional storage targets of the transmission type which include a metal mesh coated with a storage dielectric and a separate phosphor screen spaced from such transmission type storage target so that low velocity electrons are transmitted through such storage target to strike such phosphor screen in accordance with the charge image stored on such mesh target. The storage target of the present invention is discussed in detail in my copending United States patent application, Ser. No. 180,457, iiled Mar. 19, 1962, entitled, Electron Discharge Display Device, Which also discusses many other advantages of such storage target over previous mesh storage targets.

It has been found that if the low velocity holding elec'- trons emitted by the viewing or flood gun Within the image intensifier storage tube are prevented from striking the storage target at the time the high velocity writing electrons are striking the storage target to produce the charge image there-on, the storage target will store the charge image of a low intensity light image received by 33311782 Patented May 2, l?

such tube which could not be stored previously. The 10W velocity holding electrons oppose writing because they tend to drive the voltage of target areas, which have charged by the high velocity writing electrons to a potential below the first cross-over voltage on the secondary emission characteristic curve of the phosphor material, back towards the voltage of the fiood gun cathode because the secondary emission ratio in this region is less than one. Thus, the holding electrons prevent the target areas from being increased in voltage by the charging action of superimposed Writing electrons emitted in response to successive photons of light impinging upon the photocathode. However, if the low velocity holding electrons are prevented from striking the storage target to allow the potential of the charge image to build up, until it exceeds the first cross-over voltage, then the subsequent bombardment of such storage target with such low velocity electrons drives the potential of the charge image up toward the voltage of the conductive film target electrode thereby enabling bistable storage of such charge image. The image intensifier storage tube of the present invention may employ this charge image integration technique-to produce a stored charge image of a very low intensity light image received by such tube without the use of additional amplification stages. Thus, the use of the simplified phosphor dielectric layer storage target and the charge image integration technique enables the image intensifier storage tube of the present invention to be greatly simplified in construction as compared with previous image intensifier tubes.

There is some danger of destruction of the charge image as a result of regenerative optical feedback of the visible light emitted from the phosphor layer of the storage target to the photocathode which causes electrons to be emitted from the photocathode if such photocathode is made out of visible light sensitive material. In order to prevent such light feedback from destroying the charge image on the storage target, the high velocity writing electrons emitted by such photocathode are prevented from striking the storage target during the time the low velocity holding electrons bombard such target and increase the brightness of the light image emitted by the phosphor layer. In addition, the image intensifier storage tube of the present invention may be operated to provide a motion picture type visual display on the phosphor layer of the storage target. This motion picture type display may be accomplished by alternately writing and erasing a charge image on the storage target by applying voltage pulses to the conductive layer of such storage target and to the photocathode. At the same time voltage pulses may also be applied to the grids or cathodes of the flood guns to turn ofi such flood guns during writing to enable charge image integration as discussed previously.

It is therefore one object of the present invention to provide an improved image intensilier tube of simple and economical construction.

A further object of the invention is to provide an improved image intensifier tube having a secondary electron emission storage target.

Another object of the invention is to provide an image intensiiier storage tube with a storage target having a storage dielectric in the form of a senricontinnous layer of phosphor material having a thickness within a critical range over which such phosphor functions both to store the charge image produced thereon and to emit a light image corresponding to such storage image.

A still vfurther object of the present invention is t-o provide an image intensifier storage tube which produces an electrical readout signal and a visible light image corresponding to the charge image produced on the storage target of such tube.

An additional object of the invention is to provide an improved method of operation of an image intensifier storage trube in which the low velocity holding electrons are prevented from striking the storage target during the time the high velocity writing electrons emitted by the photocathode in such tube are bombarding such target to produce a charge image thereon :and until such charge image increases in voltage to a potential greater than that required for bistable storage.

Still another object of the invention is to provide an improved method of operating `an image intensifier storage tube having a phosphor viewing layer in which high velocity writing electrons are prevented from bombarding the storage target during the time the low velocity electrons strike such phosphor layer to :prevent regenerative optical feedback and to maintain the storage image produced thereon.

A further object of the present invention is to provide an improved method of operating an image intensifier storage tube in which a charge image is alternately written on and erased from the storage target of such tube to produce a motion picture visual readout display by applying voltage pulses to such target and to a photocathode in such tube.

Another object of the present invention is to provide an improved image intensifier storage tube in which a storage target including a storage dielectric in the form of a semi-continuous layer of phosphor material coated on a light transparent support member over a light transparent conductive film is employed along with a mesh electrode spaced from such storage target to prevent ions of residual gas from bombarding such storage target and to enable more uniform distribution of low velocity electrons on such target during viewing.

Other objects and advantages of the present invention will apparent from the following detailed description of certain preferred embodiments thereof and from the attached drawings of which:

FIG. l is a schematic view of one embodiment of the image intensifier storage tube of the present invention along with associated apparatus for obtaining electrical readout signals from such storage tube;

FIG. 2 shows another embodiment of the image intensifier storage tube of the present invention and apparatus for providing a motion picture visual readout display; and

FIG. 3 shows voltage pulses which may be applied to the image intensifier storage tube of FIG. 2 during the operation of such tube.

As shown in FIG. l, the present image intensifier storage tube includes a photoeathode coated on the inner surface of the curved end wall of the tube envelope 12 which is made of glass or other suitable material. A storage target 14 may be supported on the inner surface of the opposite end of the envelope spaced from such photocathode. The photocathode 10 may be made of cesium antimonide (CSSSb) or other visible sensitive photoemissive materials. Such photocathode may alternatively be made of a photoemissive material which is sensitive to invisible light, such as a mixture of silver, cesium oxide and cesium which has been activated so that it is sensitive only to infrared light. Many other suitable photoemissive materials are disclosed in the book, Photoelectricity and lts Applications, by V. K. Xworykin and E. G. Ramberg, published in 1945 by John Willey and Sons. It should be noted that yan external fluorescent screen (not shown) may be employed to convert the invisible light image to visible light so that the same Cs3Sb photocathode may be used for detecting X-ray or infrared images. A pair of conventional evaporators 16 and 18 m-ay be mounted within the U-shaped channel of an annular shield member 20 supported within the envelope 12 so that such evaporators are directed onto the curved end wall of such envelope to form the photocathode. The different materials used to form the photoemissive material of photocathode 10 may be coated on the filament wires of different evaporators which are heated to vaporize the material thereon. The vaporized material is directed onto the inner surface of the curved envelope portion by the shield member 20. It should be noted that the photocathode 16 is usually formed after the tube envelope has been evacuated because the photoemissive materials of such photocathode are highly volatile.

The storage target 14 may be the direct viewing type of simplified secondary emission storage target shown in my copending U.S. patent application, Ser. No. 180,457, discussed previously, which includes a storage dielectric layer 22 of phosphor material which stores the charge image produced thereon and emits a light image corresponding to such charge image. This dielectric layer 22 is coated over a light transparent conductive film 24 of tin oxide or other suitable conductive material on the inner surface of the face plate portion of the envelope 12. The storage dielectric 22 may be a Pl type phosphor which is applied as a semicontinuous layer having a thickness within the critical range of thicknesses over which such phosphor material will store a bistable charge image for an indefinite controllable time. As stated in my copending application, 180,457, the phosphor storage dielectric layer 22 is an integral or undivided layer, but has a sufficiently porous structure to enable secondary electrons emitted from the bombarded side of the layer to be transmitted through the layer and collected by the conductive film 24 on the opposite side of such layer. The charge image is produced on the phosphor layer 22 of the storage target 14 by high velocity writing electrons emitted from the photocathode 10 and is maintained on such storage target by low velocity holding electrons emitted from the one or more viewing electron guns 2.6. The viewing guns 26 may be of the flood gun type which flood the entire surface of the storage target with low velocity electrons so that such electrons are substantially uniformly distributed over such target, or of the scanning gun type which scan the target with a focused beam of low velocity electrons. In either case the holding electrons cause secondary emission from the storage target dielectric layer 22 so that the charge image is maintained `or stored thereon as a rbistable charge image for an indefinite controllable time. The viewing guns 26 are positioned behind the shield member 20 so that they are out of the Way of the high velocity writing electrons emitted from the photocathode, and are directed toward the storage target and away from such photocathode so that the materials emitted by such viewing guns do not destroy or contaminate the photocathode. A plurality of `focusing and accelerating electrodes 28, 30 and 32 may be provided as wall coatings of silver aqudag or other suitable conducting material on the inner surface of the glass envelope 12. The first wall coating 28 positioned between the photo cathode 10 and the viewing guns 26 functions primarily to focus the image electrons emitted from the photocathode onto the storage target 14. The second and third focusing electrodes 30 and 32 positioned generally between the viewing guns 26 and the storage target function primarily to focus the flood electrons emitted by such viewing guns onto the storage target so that such flood electrons are uniformly distributed over such storage target and strike such target substantially at right angles thereto. It should be noted that these wall coating electrodes, especially electrode 32, may also perform as collectors for the secondary emission electrons emitted from the storage dielectric layer 22 of the storage target. However, it is believed that the majority of these secondary emission electrons are collected by the conductive film target electrode 24 due to the relatively porous structure of the semicontinuous phosphor layer 22 of such target.

If electrical readout is desired, an additional electron gun 34 may be provided so that the electrons emitted by such reading gun can be directed ontomthe storage target as a focused beam of electrons. The reading gun 34 includes two pairs of horizontal and vertical deiiection plates 36 and 38 so that the reading beam may be scanned across the surface of the storage target in the manner of a TV raster pattern. The conductive film electrode 24 of the storage target 14 is connected by an electrical lead 40 extending through the envelope 32 to the Z-axis input of a remotely located monitor cathode ray tube 22. Thus, the electrical readout signal corresponding to the charge image storedon the dielectric layer 22 which is produced on the conductive film 24 by the scanning of such dielectric layer with the reading beam, is transmitted out of the envelope by lead 40 through a coupling capacitor 44 to the cathode or control grid of the monitor tube 32 in order to vary the brightness of the electron image of such monitor tube. This causes an image pattern to be displayed upon the monitor tube 42 which corresponds to the charge image stored on the storage target 14 because the horizontal and vertical raster signals applied to the horizontal and vertical deflection plates, respectively, of such monitor tube are synchronized with the raster signals applied to the deiiection plates of the reading gun 34. The saw tooth raster signals may be supplied from a common raster signal generator 46.

The photocathode and the cathode of the reading gun 34 are normally maintained at zero volts with respect to ground while the target electrode 24 of the storage target is maintained at about +3000 volts by the D C. voltage produced across a fixed resistor 4S by current iiowing to ground at one end such resistor from a source of +3500 volts through a variable resistor 50. The cathode of the viewing guns 26 may be connected to +2700 volts while the control grid of such viewing gun is connected to +2690 volts during viewing by a switch 52 and the anode of such gun is connected to about +2900 volts. This allows the fioodV electrons emitted by such viewing gun to be accelerated by a cathode to target electrode potential difference of about 300 volts before striking the phosphor layer of the storage target. As `a result, the charge image produced on the phosphor layer by the writing electrons emitted from the photocathode, is stored as a bistable charge image for an unlimited time by such fiood electrons if the voltage of such charge image exceeds the first cross-over voltage on the secondary emission characteristic curve of the phosphor material for the reasons described in my copending U.S. application, Ser. No. 180,457. Since this potential difference is insufficient to cause more than one secondary electron to be emitted from the dielectric layer 22 in those areas which have not been charged above the first cross-over voltage, for each primary electron received from the viewing gun, the flood electrons do not normally cause the entire target to be driven positive with respect to the viewing gun cathode voltage. However, this may be accomplished during the erase operation of the storage tube merely by changing the setting of the variable resistor 50 to increase the voltage on the target electrode 24 above the fade positive voltage of the phosphor to cause such target to change to an entirely written bright condition. Next the voltage on the target electrode is decreased below the retention threshold voltage of the phosphor storage dielectric so that the target returns to a completely unwritten dark condition. Then the voltage of the target electrode 24 is increased gradually above the retention threshold voltage to about +3000 volts to complete the erase operation and to enable another charge image to be formed on the storage target.

It has been found that, if the low velocity fiood electrons of the viewing guns 26 are prevented from striking the storage target 14 during the time the charge image is being Written onto the storage target by the photocathode, light images of extremely low intensity received by the photocathode 10 may be Written and stored on the storage target 14. This may be explained by the fact that the voltage of the charge image slowly builds up on the storage dielectric 22 in the manner in which a capacitor is charged, as more writing electrons bombard the storage target from the photocathode 10. However, the low velocity fiood electrons tend to oppose this increase in voltage and instead try to drive the potential of the phosphor dielectric layer down toward the voltage of the viewing gun cathode, as described previously, to erase the charge image. Thus, if the low velocity iiood electrons are prevented from striking the storage target during writing, the voltage of the charge image will increase to a value above the first cross-over voltage which is the minimum voltage which must be obtained before indefinite bistable storage is possible. Then the flood electrons may be directed onto the storage to store the charge image. This charge image integration method of operation may be accomplished by moving the switch contact 52 to the integrate position labeled INTEG. to connect the control grid of the viewing gun to about +2650 volts in order to cut ofi the flow of fiood electrons from such viewing gun during writing, and by moving such switch contact back to the VIEW position during viewing to al low such electrons to bombard the target and to store the charge image produced on such target.

As discussed above, the visible light emitted from the phosphor layer 22 of the storage target during view ing when such storage target is bombarded by flood electrons from the viewing guns could cause destruction of the charge image by regenerative optical feedback. Thus, the visible light from the phosphor layer exposes the photocathode 10 which, if sensitive to visible light, would uniformly bombard the phosphor layer with high velocity electrons which could destroy the charge image on such phosphor layer. This optical feedback problem may be eliminated, of course, by employing a photoemissive material for the photocathode which is not sensitive to light of the wave lengths emitted from the phosphor layer of the storage target. However, if it is desired to use a photocathode which is sensitive to visible light, destruction of the charge image by regenerative optical feedback may be avoided in most cases by preventing the writing electrons from striking the storagetarget during viewing. This may be accomplished by changing the potential on the photocathode to a high positive D.C. potential of about +1000 volts during the viewing periods by means of a switch 54 whose movable contact may be ganged to the movable contact of switch S2. If this is done, then any electrons emitted by the photocathode during viewing are retiected back to such photocathode by the volts potential on the first wall coating 28 which would then be 900 volts negative with respect to the photocathode. Since the majority of the light emitted by the phosphor layer 22 of the storage target is produced during viewing when the iiood electrons are bombarding such layer, changing the switch contact 54 to the VIEW position at this time would prevent substantially all of the regenerative optical feedback because very little light is emitted by the storage target when only the writing electrons are striking such target. In this regard, it should be noted that the reading beam emitted by the readout gun 34 is directed onto the storage target only during viewing so that the light produced by such reading beam scanning the phosphor layer 22 would also be ineffective in producing regenerative optical feedback since the photocathode is cut ofi at this time.

It is also possible that the envelope 12 may be provided with an L-shaped configuration and the photocathode may be positioned so that its central axis is substantially perpendicular to the central axis of the storage target. In this case the high velocity writing electrons emitted by the photocathode may be bent in a right angle onto such target by means of an electro-magnetic coil. If this were done the light emitted by the storage target of course would not be affected by the magnetic field so that it would not be bent and transmitted back to the photocathode, but would be instead transmitted straight through the side of the envelope without striking such photocathode.

The photocathode is shown to be curved in the conventional manner to compensate for pin cushion image distortion resulting from curvature of field. In addition, the storage target 14 has been shown of a smaller size than the photocathode ifi in o-rder to illustrate that the charge image on such storage target can be reduced in size with respect to the light image received by the photocathode to increase the current density of the writing striking such storage target. This increase in current density results in an increased writing speed, or in other words decreases the time required for the charge image to reach the minimum voltage necessary for storage. It should be noted that the charge image stored on the phosphor layer 22 of the storage target is a bistable charge image, rather than a half tone image which would be more desirable in some respects. However, this bistable charge image does have a decided advantage over the half' tone image in that the outline or silhouette of the object under observation is more clearly displayed on the storage target which frequently enables easier identification of such object. It has also been discovered that if the storage target 14 is irradiated by a light source of extremely high intensity positioned outside of the storage tube immediately adjacent the rear of such target, that a charge image can be formed on the phosphor layer 22 of such target directly by such intense light which is stored by the flood electrons. Thus, it is possible that the photocathode 10 may be omitted entirely from image intensifier storage tubes used to detect high intensity light images, such as those produced by strobe lamps. If this is done the phosphor material of the storage -dielectric 22 may be chosen so that it is most sensitive to the particular radiation directly received thereon. Therefore, zinc sulfide might be employed for the phosphor layer 22 to store the charge image of the X-ray pulse and to provide an extremely simple X-ray image intensifier.

As shown in FIG. 2, another embodiment of the image intensifier storage tube may be provided with a second image intensification stage 56 for more current gain, between the input photocathode 10 and the storage target 14' which are similar in construction to the corresponding elements in the tube of FIG. 1. The `second intensifier stage may include a second photocathode 58 of photoeniissive material coated on the concave surface of a curved glass support 60 which has a phosphor screen layer 62 coated on the convex side of such glass support. The glass support 60 is suitably mounted within the envelope 12 so that it holds the phosphor screen 62 in position to receive the electron image emitted by the first photocathode 1G. The light image emitted by the phosphor screen 62 causes the second photocathode 58 to emit writing electrons in an image corresponding to the light image received by the first photocathode, and such Writing electrons are then focused onto the storage layer 14 in a manner similar to that of the tube of FIG. 1.

The tube of FIG. 2 has focusing and accelerating electrodes 64, 66 and 68 made of annular metal members which are mounted on lead pins extending through the side of the envelope 12 and are connected to D.C. voltage sources of +100 volts, +10 kilovolts and +10.1 kilovolts, respectively. The first photocathode 10 may be grounded, while the second photocathode 58 may be connected to +10 kilovolts normally by the current flowing from a D.C. voltage source of +105 kilovolts to ground through the voltage divider network including a fixed resistor 70 an a variable resistor 72. Thus, the image electrons emitted by the first photocathode 10 are accelerated through a potential of about 10 kilovolts before they strike the phosphor screen layer 62. As a result of this acceleration the light emitted by such phosphor screen layer is ordinarily much brighterthan the light received by the first photocathode 1). This means that the second photocathode 58 emits more image electrons than tlie first photocathode and results in a very substantial current gain. The target electrode 24 in the storage target 14 is normally connected to a D.C. voltage of about +13 kilovolts by current fiowing from a source of +13.5 kilovolts through a voltage divider including a fixed resistor 74 and a variable resistor 76. The setting of the Variable resistor 76 determines the voltage across the fixed resistor 74 which is applied to the target electrode so that such variable resistor 76 may be used to perform an erase operation similar to that described with reference to FIG. l. Thus, the electrons emitted by the second photocathode are accelerated through a potential of about 3 kilovolts before striking the storage target to product a charge image on such storage target which is stored in a conventional manner by the low velocity ood electrons emitted by the viewing guns 26.

The two stage image intensifier storage tube of FIG. 2 has increased writing speed over the single stage tube of FIG. 1, due to the increase in current density of the writing electron bombarding the storage target. This additional gain may be necessary for detecting low intensity light images of transient events sought to be stored on the storage target 14', such as X-ray images of explosions often employed in ballistics studies. This is true in regard to transient images because the charge image integration technique described with reference to the tube of FIG. 1 would not be possible with such transient images.

A pair of evaporators 78 and 80 may be supported on the first focusing electrode 64 in front of an inwardly extending shoulder portion that functions as a shield for the evaporators so that they form the photocathode 10 on the inner surface of the curved end wall portion of the envelope in a similar manner to the evaporators of FIG. 1. In addition a second pair of evaporators 82 and 84 are positioned within an annular channel formed by an inwardly extending fiange portion of the third focusing electrode 68 in order to direct the material of such evaporators onto the concave side of the glass support 60 to form the second photocathode. Three additional focusing electrodes 86, 88 and 90 may be provided as spaced wall coatings of conducting material on the inner surface of the glass envelope 12' between the viewing guns 26' and the storage target 14. These wall coatings 36, 88 and 90 may be connected respectively to D.C. voltages of +12.9 kilovolts, +128 kilovolts and +1265 kilovolts, respectively, to focus the low velocity flood electrons emitted by such viewing guns substantially uniformly over the surface of the storage target.

A mesh electrode 92 may also be provided in the image intensifier s-torage tube of FIGS. 1 or 2 spaced in front of the phosphor dielectric layer Z2 of the storage target and connected to the third wall coating 90 or to a more positive voltage in order to re-pel positive ions of residual gas away from the storage target. The mesh electrode 92 may be in the form of a Woven wire mesh or a perforated sheet of metal which has been photoetched to provide a large number of openings therethrough which allow the passage of electrons from the second photocathode and the viewing guns to the storage target through such mesh electrode. It should be noted that the mesh electrode 92 serves the additional function of enabling vmore uniform distribution of the flood electrons over the phosphor layer of the storage target because it provides a substantially uniform equal potential surface spaced closely adjacent such phosphor layer. This equal potential surface tends to prevent adjacent positive and negative charge image areas on the phosphor storage laye-r 22 from deflecting the flood electrons in the `manner similar to the coplanar grid effect which previously produced bright and dark areas in the background illumination of such target. This is especially useful for a split screen type of storage target in which the :target electrode coating 24 is separated into two o-r more spaced electrode areas which may be connected to different voltages above or below the retention threshold voltage -to allow the portions of the phosphor l-ayer over such electrodes t-o operate independently in either a storage or non-storage mode as disclosed in my copending United States patent application, Ser. No. 214,8-77 tiled Aug. 6, 1962, and entitled, Storage Tube.

The -cathodes of the viewing guns 26' may be each connected to a D.C. voltage of about +12.7 kilovolts while the control grids of such viewing guns may be normally connected to a D.C. voltage of +12.69 kilovolts which is about 10 volts negative with respect to such cathodes. The bias voltage on the control grids may be supplied by the D.C. voltage d-rop across a fixed resistor 94 forming a voltage divider with a variable resistor 96, which is connected between a source of +13 kilovolts and ground so that the viewing gun is normally biased conducting. However, as has been discussed previously with regard to the tube of FIG. 1, it has been found desirable to turn off the viewing guns and prevent .the kflood electrons from striking the storage dielectric layer 22 during writing to increase the writing speed of the tube 4and in the case of continuous or repetitive images to allow charge image integration. This may be accomplished by applying a large negative voltage pulse to the control grid of the viewing gun or a large positive Voltage pulse to its cathode in order to bias such viewing gun into cutoff. Thus the control grid may be connected to a pulse generator (not shown) at an input terminal 100 to allow such pulse generator to apply suitable negative rectangular pulses 102 to such control grid during writing to cut off the Viewing guns 26'. The voltage of the pulses 102 which enable charge image integration may be about -50 volts so that the total voltage applied to the control grid is sufcient to prevent tiood electrons from being emitted by the viewing guns.

In a similar manner an erase pulse 104 may be applied to the target electr-ode layer 24' in order to erase the charge image previously stored on the phosphor layer of such storage target. Thus, erasure may be accomplished by pulsing rather than by varying the setting of variable ristor 7'6, lmerely by connecting -a suitable pulse generator (not shown) to an input terminal 108 which is connected to the lower end of such variable resistor. This erase pulse may be of the type shown in FIG. 3 in which the voltage rapidly rises to about +200 volts to drive the target electrode 24 to +13.2 kilovolts or +500 volts with respect to the viewing gun cathode, where it is maintained const-ant until the storage target fades positive, and then drops in voltage in a negative direction to -300 volts to apply +12.7 kilovolts to the target, or zero volts with .respect to the cathode, which is below the retention threshold voltage lof the target to establish a +12.7 kilovolts charge on the rear surface of the phosphor layer 22. Thereafter, such erase pulse returns slowly to zero along an exponential R.C. time constant curve to raise the voltage on a target electrode lback to its normal of +13 kilovolts without raising the potential on the rear surface of the phosphor layer.

In addition, regenerative optical feedback may be prevented in the tube of FIG. 2 by applying a positive rectangular viewing pulse 110 to the second photocathode 58 by connecting a suitable pulse generator (not shown) to .an input terminal 114 connected to the upper end of resistor 70. This positive pulse prevents the Writing electrons emitted by Ithe second photocathode from passing through the focusing member 618 during viewing because such focusing member is then sufficiently negative with respect to the second photocathode to cut off such photocathode. It may also be desirable Ito provide 4a thin elect-ron transparent, light reflecting aluminum coating 11'6 over the phosphor screen laye-r 62 of the second intensifier stage to prevent the -light emitted by such phosphor screen layer from being transmitted to the first photocathode 10'. The integrate pulse 102, the viewing pulse and the erase 'pulse 104 are shown in time relationship in FIG. 3.

It will be obvious to those having ordinary skill in the art that various changes may 'be made in the details of the above described preferred embodiment of the present invention without departing from the spirit of the invention. Therefore, the scope of the present invention should only be determined by the following claims.

I claim:

1. An image intensifier device, comprising:

writing means including a photocathode, for emitting electrons in response to the irradiation of said photocathode with light;

storage means including a bistable storage target for storing the charge image produced on said target by the Ibombardment of said target with high velocity electrons from said writing means; and

Imeans for bombarding said target with low velocity electrons to cause bistable storage of a charge image on said target corresponding to a light image received by said photocathode.

2. An image intensier device, comprising:

Writing means including a photocathode for emitting electrons in response to the irradiation of said photocathode with light;

storage means including a storage ltarget having .a storage dielectric of phosphor material for storing the charge image produced on said target by the bombardment of said target with high velocity electrons from said writing means -and for emitting a light image corresponding lto said charge image;

focusing means for focusing said 4high velocity electrons onto said target to produce -a charge image on said target corresponding to a light image received by said photocathode; and

means for bombarding the phosphor storage dielectric with low velocity electrons to store said charge image on said target as a bistable image fo-r a controllable time.

3. An image intensifier device, comprising:

writing means including a photocathode for emitting electrons in response to the irradiation of said photoi cathode with light;

storage means including a bistable sto-rage target having a storage dielectric of phosphor material for storing a charge image produced by the bombardment of said target with high velocity electrons from said writing means and for emitting a light image corresponding to said charge image;

focusing means for focusing electrons onto said target means to produce a charge image on said target corresponding to a light image received by said photocathode;

means for bombarding the phosphor storage dielectric with low velocity electrons to store said charge image on said traget as a bistable image; and

readout means for producing an electrical readout signal corresponding to said charge image.

4. An image intensifier device, comprising:

writing means including a photocathode for emitting electrons in response to the irradiation of said photocathode with light:

storage means including a storage target having a storage dielectric layer of phosphor material coated over a conductive surface of a light transparent support member, for storing a charge image produced by the bombardment of said storage dielectric layer with electrons and for emitting a light image corresponding to said charge image;

focusing means for focusing the electrons emitted lby said cathode means onto said storage dielectric means to produce a charge image on said target corresponding to a light image received by said photocathode; and

means for bom'barding the storage dielectric layer with low velocity electrons to store said charge image on said target as a bistable image.

5. An image intensifier storage tube, comprising:

an evacuated envelope;

-a bistable storage target means including a storage dielectric of phosphor material mounted within said envelope;

writing lmeans including a photocathode mounted within said envelope spaced from said storage target, for causing a charge image lto be produced by high velocity elec-trons on said storage target corresponding to the light image received by photocathode;

viewing means including a flood electron gun supported Within said envelope to direct low velocity electrons onto the storage target in order -to cause said charge image to be stored bistably on said storage target; and

means for preventing the low velocity electrons of said ood gun from bombarding the dielectric layer of said storage target at the same time that the hig-h velocity electrons are striking said storage dielectric to form said charge image until after the potential of the charge image exceeds a minimum potential necessary for bistable storage.

6. An image intensier storage tube, comprising:

an evacuated envelope;

a bistable storage target means mounted within said envelope including a light transparent support member of insulating material, a light transparent target electrode layer of conducting material coated on one side of said support member, and a 4storage dielectric layer of phosphor material supported on said support member over said target electrode layer, said storage dielectric layer being a semicontinuous layer of phosphor having a thickness within the range of thicknesses over which said phosphor layer can store a bistalble charge image thereon for an indefinite controllable time and will emit a light image corresponding to said charge image;

writing means including a photocathode mounted within said envelope spaced from saidl storage target, for causing a charge image to be produced on said dielectric layer by the bombardment of high Velocity electrons corresponding to the light image received by said photocathode;

a viewing electron gun supported within said envelope to direct low velocity electrons onto the storage dielectric layer `in order to cause said charge image to be stored tbistably on said dielectric layer; and

a plurality of spaced focusing electrodes mounted within said envelope.

7. An image intensifier storage tulbe, comprising:

an evacuated envelope;

a storage target mounted 'within said envelope including a light transparent target electrode layer of conducting material and a storage dielectric layer of phosphor material coated over said target electrode layer as a semicontinuous layer of phosphor having a thickness within the range of thicknesses over which said phosphor can store a charge image produced thereon for an indefinite controllable time and will emit a light image corresponding to said charge image;

a photocathode mounted within said envelope spaced from said storage target to cause a charge image to =be produced on said dielectric layer by the bombardment of high velocity electrons corresponding to the lightimage received *by said photocathode;

a viewing electron gun supported within said envelope to direct low velocity electrons substantially uniformly onto the storage `dielectric layer in order to cause said charge image to be stored on said dielectric a plurality of spaced focusing electrodes mounted withl2 in said envelope between said photocathode and the storage target and between said viewing gun and said storage target; and

means for preventing the low velocity electrons of said viewing gun from bombarding the dielectric layer of said storage target at the same time that the high velocity electrons are striking said dielectric layer and producing said charge image in order to enable the charge image to increase in voltage until it eX- ceeds the minimum potential necessary for storage before directing said low velocity electrons onto said dielectric layer.

8. An image intensifier storage tube, comprising:

an evacuated envelope;

a storage target mounted within said envelope including a light transparent target electrode layer of conducting material 4and a storage dielectric layer of phosphor material coated over said target electrode layer as a semicontinuous layer of phosphor having a thickness within the range of thicknesses over which said phosphor can store a charge image produced thereon for an indefinite controllable time and will emit a light image corresponding to said charge image;

a photocathode mounted .withn said envelope spaced from said storage target to cause a charge image to be produced on said dielectric layer by the lbombardment of high velocity electrons corresponding to the light Vimage received by said photocathode;

a viewing electron gun supported within said envelope to direct low velocity electrons substantially uniformly onto the storage dielectric layer in order to cause said charge image to be stored on said dielectric layer;

a plurality of spaced focusing electrodes mounted within said envelope fbetween said photocathode and the storage tar-get -and between said viewing gun and said storage target;

a mesh electrode supported inside said envelope adjacent said storage target to repel ions away from said storage target and to cause said low velocity electrons to be more uniformly distributed over the surface of said dielectric layer;

a readout electron gun supported within said envelope to scan said storage dielectric layer with a beam of electrons to produce an electrical readout signal on said target electrode layer corresponding to the charge image on said dielectric layer; and

means for preventing the low velocity electrons of said viewing gun from lbombarding the dielectric layer of said storage target at the same time that the electrons produced by said photocathode are striking said dielectric layer and forming said charge image in order to enable the charge image to increase in voltage until it exceeds the minimum potential necessary for storage before directing said low velocity electrons onto said dielectric layer.

9. An image intensifier storage tube, comprising:

an evacuated envelope;

a bistable storage target means mounted within said envelope including a light transparent support memiber of insulating material, a light transparent target electrode layer of conducting material coated on one side of said support member, and a storage dielectric layer of phosphor material supported on said support member over said target electrode layer, said storage dielectric layer being an integral Alayer of phosphor material having a suiciently porous structure to ena-ble secondary electrons emitted vfrom one side of the phosphor layer to be transmitted through the phosphor layer and collected `by the target electrode layer on the opposite side of such phosphor layer so that said phosphor layer can store a bistable charge image thereon for an indefinite controllable time and will emit a light image' corresponding to said charge image;

13 14 Writing means including a photocathode mounted with- References Cited by the Examiner electric layer by the 'bombardment of high velocity 2,817,781 12/1957 Sheldon 315-11 X electrons corresponding to the light image received 5 2,853,648 9/1958 Then@ 315' 13 X by said photoeathode; and 2,941,100 6/1960 Farnsworth 315-13 X a viewing electron gun supported Within said envelope to direct low velocity electrons onto the storage di- DAVID G- REDINBAUGH, Pflmdl'y Exmlnefelectric layer in order to cause said charge image to T A GALLAGHER ASSI-stam Examiner be stored 'bistalbly on said dielectric layer. 10

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,317,782

May 2, 1967 Robert H. Anderson It is hereby certified that error appears in the above numbered patent requiring correction and that. the said Letters Patent should read as corrected below.

Column 2, line 3, after "have insert been column 3, line 34, after "will" insert be line 55, for "CS3Sb" read Cs3Sb same line 55, after "visible" insert light line 62, for "Xworykin" read Zworykin line 63, for

"Willey" read Wiley column 4,

"Aquadag" column 5, line 7,

line 51,. for "aqudag" read column 8,

line 27, for "integration" read integration" column l0, line 57, for "traget" read target line 72, for "means", second occurrence, read layer column ll, line I3, after "by" insert said line 24, for "a" read the Signed and sealed this 21st day (SEAL) Attest EDWARD M.PLETCHER,JR. Attesting Officer of November 1967.

EDWARD J BRENNER Commissioner of Patents 

1. AN IMAGE INTENSIFIER DEVICE, COMPRISING: WRITING MEANS INCLUDING A PHOTOCATHODE, FOR EMITTING ELECTRONS IN RESPONSE TO THE IRRADIATION OF SAID PHOTOCATHODE WITH LIGHT; STORAGE MEANS INCLUDING A BISTABLE STORAGE TARGET FOR STORING THE CHARGE IMAGE PRODUCED ON SAID TARGET BY THE BOMBARDMENT OF SAID TARGET WITH HIGH VELOCITY ELECTRONS FROM SAID WRITING MEANS; AND MEANS FOR BOMBARDING SAID TARGET WITH LOW VELOCITY ELECTRONS TO CAUSE BISTABLE STORAGE OF A CHARGE IMAGE ON SAID TARGET CORRESPONDING TO A LIGHT IMAGE RECEIVED BY SAID PHOTOCATHODE. 